Chemistry Labs

Grade 12

Air, water and soil pollution

Pollutants move among air, water and soil, where they can react, accumulate or harm organisms. Learn their sources, effects and chemical controls, from acid-rain scrubbing to catalytic converters.

1 Lower secondary2 Upper secondary3 Undergraduate4 Advanced5 Research

IntuitionPollution connects three environments

A pollutant released to air may dissolve in rain, settle on soil, enter a stream and accumulate in organisms. Preventing release is often more effective than cleaning up after contamination spreads.

Adjust diffusion/mixing D. Darker colors indicate higher concentration; larger D dilutes each plume but spreads it farther across the river and soil.

SchoolSchool level: pollutants and their transformations

Definition: Primary and secondary pollutants

A primary pollutant is emitted directly (for example, SO₂ or CO). A secondary pollutant forms in the environment from reactions among emissions and natural constituents; ground-level ozone and sulfate aerosol are examples.

Common pollutants, sources and effects
PollutantSources and key concerns
SO₂; NOₓSulfur-containing fuel combustion and high-temperature engines/furnaces; acid deposition and respiratory irritation
CO; particles (PM₂.₅/PM₁₀)Incomplete combustion, soot and road dust; CO impairs oxygen transport, while fine particles penetrate deep into lungs
VOCs; heavy metals (Pb, Hg, Cd, As)Evaporation and combustion; mining, metallurgy and waste. Some VOCs form photochemical smog; metals can be toxic and persistent
Nutrients; pesticidesFertilizer and wastewater runoff adds N and P and can trigger eutrophication; persistent pesticides may bioaccumulate

Sulfur in fuel burns to SO₂. In air, oxidation and hydration can produce sulfuric acid and sulfate particles; nitrogen oxides can form nitric acid. These acids lower rain and soil pH and can leach nutrient ions or mobilize toxic aluminium.

S+OX2→SOX2;2 SOX2+OX2→catalyst2 SOX3;SOX3+HX2O→HX2SOX4\ce{S + O2 -> SO2; 2 SO2 + O2 ->[catalyst] 2 SO3; SO3 + H2O -> H2SO4}
NX2+OX2→high T2 NO;2 NO+OX2→2 NOX2;4 NOX2+OX2+2 HX2O→4 HNOX3\ce{N2 + O2 ->[high\ T] 2 NO; 2 NO + O2 -> 2 NO2; 4 NO2 + O2 + 2 H2O -> 4 HNO3}

Sulfurous smog is associated with SO₂ and particles in cool, humid air; photochemical smog forms in sunlight when NOₓ and VOCs generate ozone and other oxidants. Ground-level ozone is harmful even though stratospheric ozone is protective.

Nitrate and phosphate runoff can trigger eutrophication: algal growth increases, then decomposition consumes dissolved oxygen. Low oxygen can kill fish. Excess fertilizer is not the only cause; wastewater and altered water flow also matter.

CX6HX12OX6+6 OX2→6 COX2+6 HX2O\ce{C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O}

Example: Scrubbing sulfur dioxide

A flue-gas scrubber uses limestone. What mass of pure CaCO₃ is stoichiometrically needed to capture 6.40 kg SO₂? Use M(SO₂)=64.1 and M(CaCO₃)=100.1 g mol⁻¹.

Solution

The net 1:1 reaction is CaCO₃ + SO₂ + ½O₂ + 2H₂O → CaSO₄·2H₂O + CO₂. n(SO₂)=6400/64.1=99.8 mol, so n(CaCO₃)=99.8 mol and m≈9.99 kg. Real plants need excess reagent and account for incomplete contact.

Three-way catalytic converters, typically containing Pt/Pd and Rh, oxidize CO and unburned hydrocarbons while reducing NOₓ near the stoichiometric air–fuel ratio. Diesel systems may instead use selective catalytic reduction with ammonia.

2 CO+OX2→2 COX2;2 NO+2 CO→NX2+2 COX2\ce{2 CO + O2 -> 2 CO2; 2 NO + 2 CO -> N2 + 2 CO2}

Example: Comparing rain acidity

How many times greater is [H⁺] at pH 4.0 than at pH 5.6, a typical value for clean rain equilibrated with atmospheric CO₂?

Solution

[H⁺]=10⁻ᵖᴴ mol L⁻¹, so the ratio is 10^(5.6−4.0)=10^1.6≈40. Clean rain is mildly acidic because dissolved CO₂ forms carbonic acid; acid deposition is more acidic due mainly to sulfuric and nitric acids.

UndergraduateUniversity glimpse: emissions, transport and control

A simple well-mixed box balance is dC/dt = E/V − kC − Qout C/V + Qin Cin/V. It tracks emission E, volume V, chemical or depositional removal k, and ventilation; a diffusion map is a spatial picture of the same competition between spreading and removal.

Environmental control combines source reduction, capture and transformation. Limestone flue-gas desulfurization transfers SO₂ into a solid sulfate; converters transform toxic gases into less harmful products, but do not remove all emissions or eliminate the need for maintenance.

References